The first functional version of the firmware is ready! Now I can use/generate 10 different passwords.

To avoid non intentional password reprogramming the SEND key must be held pressed, as a safeguard, while tapping the Caps Lock key.

Next step will be on how to upload legacy passwords, but I am prone to use USBaspLoader together with AVRDude to write the passwords directly to the EEPROM space of the AVR.